The invention relates to a casting wheel for the continuous casting of strips of metal, preferably strips of ferrous metal within a thickness range between 1 mm and 12 mm, comprised of a core and a sleeve made of heat-conductive material, which is shrunk on the core, circumferential cooling ducts being located between the core and the sleeve, which are connected with coolant supply and discharge lines.
Arrangements for the production of strips of ferrous metal with near-net-shape cross section in a continuous casting process are known. In this process, liquid steel is continuously fed from an intermediate vessel onto a rotating casting wheel in a desired layer thickness and taken from the casting wheel after it has fully or partly solidified (single-wheel strip casting). Near-net-shape strips can also be produced by feeding the liquid steel into a liquid sump formed by two casting wheels rotating in opposite directions and by side walls, the molten metal solidifying on the cooled casting wheel surface and forming two strand shells which are connected to form a casting strand in the smallest cross section between the two casting wheels, which has a defined thickness as a function of the distance between the two casting wheels (twin-wheel strip casting).
A casting wheel of the type applied in twin-wheel strip casting is known from IT-PS 1 255 817. This casting wheel is comprised of a core and a sleeve shrunk on the core, in which circular, discontinuously circumferential cooling ducts are installed. Coolant is centrally fed through the central shaft to the circular cooling ducts via radially arranged collecting mains and analogously discharged from there. The thickness of the sleeve changes at the transition between the collecting mains and the cooling ducts, which is due to the discontinuously circumferential cooling ducts which serve to separate coolant being fed from coolant being discharged and which leads to different radial and axial deformations of the sleeve at this point during operation, which are detrimental both to the production process and to the product itself. Deformations cause thickness variations of the product. Moreover, as the sleeve is alternately heated and cooled in accordance with the rotation of the casting wheel, the sleeve slowly twists in relation to the core, which cannot be completely remedied even if a form-fitting anti-rotation device is installed.
This torsion may lead to a short-circuit flow between the supply line and the discharge line to a great extent, which must be prevented by all means.
A solution where these disadvantages are eliminated is known from DE-OS 196 12 202, which proposes a generic casting wheel featuring a sleeve with circular, continuous cooling ducts installed in the sleeve, which is shrunk on a core. Starting from the collecting mains, coolant is fed into the cooling ducts on both sides and, having passed through 180xc2x0, exits the casting wheel at the opposite side and flows into collecting mains. This solution has the essential disadvantage that the amount of coolant required for cooling doubles while the flow velocity required to achieve the desired cooling effect and resulting from branching of the coolant flow is kept constant. The increased demand for coolant can be counteracted by using it twice, as stated in DE-OS 196 12 202 for a special embodiment. However, this method involves the disadvantage of a complicated core design. Moreover, axial zones are formed due to this complex coolant flow guidance, with coolant of different temperatures flowing through the cooling ducts adjacent to these zones. As a result, the mean casting wheel temperature clearly varies also in these transition regions or in the different axial zones, and particularly a jump of temperature occurs at the transition from one axial zone to the next one. These temperature changes represent a heavy mechanical load on the core and on the sleeve and also impair the cast strip of metal, since uniform thermal conditions in axial direction are absolutely essential in order to achieve a high strip quality. In addition, the flow may not be guided uniformly enough into the circular cooling ducts, which occurs, for example, in case of contamination.
The object of the invention is to avoid these disadvantages and difficulties and to create a casting wheel of the type described at the beginning, which provides for a uniform dissipation of heat from the sleeve at minimized coolant consumption and clearly defined flow conditions and also allows for the thermal tendency of the sleeve toward offset relative to the core. Another object of the invention is to provide a casting wheel of simple design that requires simple manufacturing methods, whose sleeve is rotationally symmetric and whose thermal expansion is mechanically unimpeded.
This technical problem is solved by a casting wheel of the type described at the beginning in that the guidance of coolant from the basically radial supply line into the cooling ducts and the guidance of coolant from the cooling ducts into the basically radial discharge line is caused by a guiding element that can be installed in the cooling ducts.
In any embodiment of the invention, one guiding element is allocated to each cooling duct. A preferred embodiment which is easy to mount is characterized in that a common guiding element is allocated to several adjoining cooling ducts.
The guiding element is comb-shaped, the width and depth of the individual teeth largely corresponding to the width and depth of the cooling ducts. An embodiment easy to manufacture is achieved by designing the guiding element with individual plate-shaped elements alternately forming teeth and intervals and held together by a connecting element, preferably a bolt, which threads with the individual plate-shaped elements.
In a preferred embodiment of the invention, a defined gap is set between the sleeve and the guiding element, particularly between the bottom of the cooling ducts and the face of the guiding element. This defined gap allows a desirable leakage flow between the inflow area and the outflow area whose extent is predetermined, which leads to a highly axisymmetrical behavior of the sleeve. Particularly favorable conditions are attained by dimensioning the gap as a function of the length of the guiding element, which varies in depth, in such a way that the mean velocity of coolant flow in the gap corresponds to the coolant flow velocity in the other regions of the cooling ducts. The gap between the guiding element and the sleeve is thus dimensioned so that in the area of the guiding element similar cooling conditions are attained as occur in any other region of the sleeve. For a gap length of 50 mm, gap widths ranging from approx. 0.3 to 0.8 mm are calculated depending on the pressure difference and mean velocity of coolant flow (4 to 15 m/s).
The coolant is guided from the radial supply line into the circular cooling ducts and vice versa in a way that favorable flow conditions are achieved by designing the teeth of the guiding element progressively divergent in radial direction toward the bottom of the cooling ducts.
In another embodiment, the teeth of the guiding element are preferably degressively convergent in radial direction. Since a very narrow face of the guiding element is opposite the bottom of the cooling ducts in this case, the risk of the gap between these two components becoming clogged is minimized or negligible, and the danger of great asymmetric deformations resulting from nonaxisymmetrical thermal conditions is also kept at a minimum.
In another embodiment, the thermal conditions in the sleeve are additionally equalized by staggering individual guiding elements or groups of several guiding elements arranged side by side in the direction of the longitudinal axis of the casting wheel by an angle in relation to the longitudinal axis of the casting wheel. A casting wheel of simple structure is obtained by aligning all guiding elements parallel to the longitudinal axis of the casting wheel.
In order to facilitate mounting and positioning, the guiding element is connected with the core by means of a plug connection. The guiding element is preferably connected with a partition between the coolant supply line and the coolant discharge line by means of a plug connection. The partition forms part of the core. According to the invention, the plug connection is essentially formed by a groove located parallel to the longitudinal axis of the casting wheel.
The guiding element is preferably made of a material that is equally heat-conductive as or less heat-conductive than the sleeve so as to reliably prevent any mounting problems or seizing in the cooling duct.